Thermoplastic fluorocopolymers and the fluoromonomers used for their preparation

ABSTRACT

The present invention relates to fluoropolymers with improved temperature resistance and dimensional stability in the heat based on fluoroethylenes, using perfluoro-(cycloalkyl-vinyl ethers) and optionally other monomers as comonomers. The invention also relates to the perfluor-(cycloalkyl-vinyl ethers) which impart the advantageous thermal properties to the fluoropolymers according to the invention.

The present invention relates to fluoropolymers with improvedtemperature resistance and dimensional stability in the heat based onfluoroethylenes, for which perfluoro-(cycloalkyl-vinyl ethers) andoptionally other monomers are used as comonomers. The present inventionalso relates to the perfluoro-(cycloalkyl-vinyl ethers) which impart theadvantageous thermal properties to the fluoropolymers according to theinvention.

Fluoropolymers are used in technology whenever special properties arerequired, such as low surface tension, high resistance to chemicals,oils or solvents or extreme resistance to (heat) ageing.

Polytetrafluoroethylene (PTFE), which is the most mass producedsynthetic resin in the field of fluoropolymers, combines theabove-mentioned properties to an eminent degree but, as is well known,cannot be processed thermoplastically. An improvement in thethermoplastic processibility is obtained by the introduction ofcomonomers which lower the viscosity of the polymer above the softeningpoint (melting point in partially crystalline systems) and thus improvethe melt flow. Hexafluoropropene and perfluorinated acyclic alkyl-vinylethers (U.S. Pat. No. 3,180,895) are examples of such comonomers but theintroduction of such comonomers in most cases lowers the softening pointof the copolymer so that the thermoplastic processibility is obtained atthe expense of the temperature stability of the polymer.

Other fluorine-containing homopolymers, such as polyvinylidene fluorideor polychlorotrifluoroethylene, can be processed thermoplastically butowing to their low fluorine content they frequently do not reach thelevel in the above mentioned properties which is achieved by the mosthighly fluorinated (co)polymers.

It is an object of the present invention to provide newthermoplastically processible fluorocopolymers with increased resistanceto temperature and dimensional stability in the heat based onfluorine-containing ethylenes as one of the comonomers.

It has been found that thermoplastic copolymers with increasedtemperature resistance and dimensional stability in the heat can beproduced by the copolymerisation of fluorine-containing ethylenes withperfluoro-(cycloalkylvinyl ethers) corresponding to the followingformula I. ##STR1## n=0, 1 or 2, m=3, 4 or 5.

Other comonomers may also be used for copolymerisation to modify theproperties of the fluorocopolymers according to the invention. Theseother comonomers include in particular straight chain or branched C₃ -to C₈ -alkenes having at least one fluorine atom or fluorine-freeethylene or propylene.

The present invention thus relates to thermoplastic fluorocopolymersobtained by the copolymerisation of

a) from 99.5 to 50 mol-% of at least one ethylene having 1 to 4 fluorineatoms and

b) from 0.5 to 50 mol-% of at least one perfluoro-(cycloalkyl-vinylether) corresponding to formula I and

c) from 0 to 40 mol-% of at least one other comonomer selected fromethylene, propylene and straight chain or branched C₃ - to C₈ -alkenescontaining at least one fluorine atom.

The molar ratio of fluoroethylenes to perfluoro-(cycloalkyl-vinylethers) in the fluorocopolymers according to the invention is preferablyfrom 1 to 50, most preferably from 2 to 20.

The ethylenes containing at least one fluorine atom (component a)) maybe tetrafluoroethylene, trifluoroethylene, vinylidene fluoride,monofluoroethylene or chlorotrifluoroethylene. Tetrafluoroethylene andvinylidene fluoride are preferred.

Among the perfluoro-(cycloalkyl-vinyl ethers) of formula I, those inwhich n=0 are preferred. Perfluoro-(cyclopentyl-vinyl ether), i.e. thecompound according to formula I in which n=0 and m=4, is particularlypreferred for the invention.

The perfluoro-(vinyl-cycloalkyl)-ethers according to the invention areprepared from 2-cycloalkoxy-propane carboxylic acid fluorides(obtainable according to U.S. Pat. No. 3,274,239). They may be obtainedby conversion into their alkali metal salts followed by decarboxylationat 170° to 250° C., as described in U.S. Pat. No. 3,274,239. If thisprocess is employed, however, considerable quantities of1,1,1,2-tetrafluoro-ethylcycloalkyl ethers are formed as by-productswhich are difficult to remove and only with high losses in yield.Residues of such by-products seriously interfere with thecopolymerisation and lead to fluorocopolymers with unsatisfactorymechanical and thermal properties. The said difficulties of producingperfluoro-(cycloalkyl-vinyl ethers) in a form suitable forcopolymerisation with fluoroethylenes must be regarded as the reason whythey have hitherto not been used in fluoropolymers.

It has now been found that 2-cycloalkoxy-propane carboxylic acidfluorides can be converted into a form ofperfluoro-(vinyl-cycloalkyl)-ethers suitable for the preparation offluoro copolymers by employing a process analogous to that disclosed inEP-A 260 773.

According to the invention, the perfluoro-(cycloalkylvinyl ethers)corresponding to formula (I) to be used for the copolymerisation areprepared from the corresponding 2-cycloalkoxy-propane carboxylic acidfluorides corresponding to the following formula II ##STR2## by slowlyheating these to a temperature of from 110°-140° C. in a suitablesolvent containing a salt-forming agent in the presence of a catalyticquantity of N,N-dimethylformamide, withdrawing a distillatecorresponding to a condensation temperature of from 60° to 90° C. at atemperature of 100° to 120° C. and redistilling the distillate at normalpressure.

The solvent used is preferably diglyme (diethylene glycoldimethylether). The salt-forming agent used is preferably anhydrouspotassium carbonate. The salt-forming agent is preferably used in a1.1-1.3 molar excess, based on the perfluoro-2-cycloalkoxy-propanecarboxylic acid fluoride. Dimethylformamide is preferably used inquantities of from 1-5% by weight, based on the solvent. It is essentialto operate under absolutely anhydrous conditions.

To produce the copolymers according to the invention, the known radicalprocesses known for the copolymerisation of fluorine-containing monomersmay be employed. The copolymerisation may be carried out in solution,suspension or emulsion (U.S. Pat. No. 2,968,649; U.S. Pat. No.3,051,677; U.S. Pat. No. 3,053,818; U.S. Pat. No. 3,331,823; U.S. Pat.No. 3,335,106 containing examples of suitable reaction media and generalreaction conditions). Basically well known compounds suitable for theparticular reaction medium are used for initiating the radicalcopolymerisation. Thus the compounds used for solution or suspensionpolymerisation may be organic, oil-soluble peroxides which may also befluorinated, such as benzoyl peroxide, trifluoroacetyl peroxide ororganic soluble azo compounds such as azo-bis-isobutyronitrile. Theinitiators used for emulsion polymerisation, which is the preferredmethod for producing the copolymers according to the invention, arewater-soluble inorganic percompounds such as persulphates, perborates,percarbonates, etc., generally in the form of their sodium or ammoniumsalts.

If relatively low temperatures are employed for polymerisation,decomposition accelerators, generally reducing agents, must be used inaddition, depending on the polymerisation temperature and on thedecomposition constant of the initiator. The following may be used forthis purpose: Sulphur compounds such as sodium sulphite, sodiumpyrosulphite or Rongalit C (sodium formamidine sulphinic acid) ororganic reducing agents such as ascorbic acid, metal salts such asiron(II) or cobalt(II) salts, organometallic compounds, etc.

The reaction temperatures for copolymerisation are from -15° C. to +120°C., preferably from 20° to 90° C.

Chain transferers such as methanol, isopropanol, isopentane, ethylacetate, diethylmalonate or carbon tetrachloride may be used ifnecessary for adjusting the molecular weight of the polymers as they arebeing produced.

It is a further characteristic of the process according to the inventionthat copolymerisation is carried out at an elevated pressure. Thispressure should be at least 5 bar but need not exceed 100 bar.

From 5 to 65 bar is a preferred range for the process according to theinvention.

The copolymers according to the invention may be produced batchwise butare preferably produced by a semi-continuous or a continuous process.

Linear copolymers having molecular weights of from 10³ to 10⁶ g/mol areobtained.

The invention will now be described in more detail with the aid of thefollowing Examples:

EXAMPLE 1

a) Preparation of Perfluoro-2-cyclopentoxy-propane carboxylic acidfluoride

150 g (2.59 mol) of annealed potassium fluoride are suspended in 500 mlof diglyme and 456 g (2.0 mol) of octafluorocyclopentanone areincorporated by condensation at about 5° C. within 1 hour with stirring.When the slightly exothermic reaction has died down, the reactionmixture is again cooled to about 5° C. and hexafluoropropene oxide isintroduced so rapidly that only slight reflux occurs at the dry icecondenser. When 330 g (1.99 mol) of hexafluoropropene oxide have beenintroduced (about 60 g/h), stirring is continued for 1 hour at 5° C. andovernight at room temperature.

The product is then distilled off at reduced pressure (up to 20 mbar)(bp. up to 60° C.) and a diphasic distillate is collected in the cooledreceiver. The upper phase contains about 90% of diglyme and the lowerphase (about 80% of product) may be transferred to the next stagewithout further purification.

To characterise the crude product (lower phase) and determine the yield,it is redistilled through a short column at normal pressure.

b) Conversion into Perfluorocyclopentyl Vinyl Ether

175 g (1.26 mol) of pulverulent anhydrous potassium carbonate and 3 mlof dimethylformamide are introduced into 200 ml of absolute diglyme and394 g (1.0 mol) of perfluoro-2-cyclopentoxy-propane carboxylic acidfluoride are added dropwise at room temperature in 1 hour. The reactionmixture is slightly exothermic (up to about 35° C.). When all thereactants have been added together, the reaction mixture is slowlyheated to 60° C., 80° C. and 110° C. (in each case about 1 hour, lightreflux and liberation of CO₂ from 80° C. upwards, and a distillate canbe drawn off via a bridge, starting at 110° C.). The sump temperature israised to a maximum of 130° C. and the head temperature of thedistillate fluctuates from 65°-85° C. The crude product (280 g) isredistilled through a 40 cm column at normal pressure:

Main fraction Bp₁₀₁₃ : 80°-82° C.

Yield: 255 g (78% of theory).

(GC-)MS: m/_(e) =328 (molar peak).

¹⁹ F-NMR: δ=

-35.0 ppm (2d, 1F, J_(F-F) =83 and 65 Hz, CF=C--O cis);

-42.1 ppm (4 m, 1F, J_(F-F) =111.83 and 6 Hz, CF=C--O trans);

-51.1 ppm (pseudo-quartet (higher order AA'BB'-system) , 4F, "J"=258 Hz,2CF₂ );

-52.5 ppm (pseudo-quartet (higher order AA'BB'-system), 4F, "J"=258 Hz,2CF₂ );

-56.5 ppm (4m, 1F, J_(F-F) =111.65 and 6 Hz, =CFO) and

-58.8 ppm (m, 1F, OCF) (against external CF₃ COOH).

The proportion of 1,1,1,2-tetrafluoroethylperfluoro-cyclopentyl vinylether is less than 2% according to GC.

The perfluoro-(cyclopentyl vinyl ether) prepared according to thisExample was used in polymerisation Examples 3, 4, 5 and 6.

COMPARISON EXAMPLE 1b

Conversion analogous to the process according to U.S. Pat. No.3,274,239.

394 g (1.0 mol) of perfluoro-2-cyclopentoxy-propane carboxylic acidfluoride are dissolved in 300 ml of dioxane and made alkaline to phenolphthalein with 40.5 g of sodium hydroxide in 100 ml of H₂ O. The solventis then removed in a water jet vacuum and the salt left behind is dried.The dry salt is then decomposed by heat (170°-250° C.) in an oil pumpvacuum (0.3 mbar) and the reaction gases are condensed in a trap whichis cooled to -78° C.

This crude product (295 g) has a GC purity of 55% (45%1,1,1,2-tetrafluoroethyl-trifluorovinyl ether) and cannot be enrichedbeyond 90% by distillation.

A polymerisation experiment carried out with the crude product and withthe enriched product resulted in very low yields of discolouredproducts, as shown in Comparison Example 6.

EXAMPLE 2

Preparation of perfluorocyclobutyl vinyl ether

69 g (0.2 tool) of perfluoro-2-cyclobutoxy-propane carboxylic acidfluoride (prepared from perfluorocyclobutanone and hexafluoropropeneoxide analogously to Example 1a) , the perfluorocyclobutanone havingbeen obtained according to J. Chem. Soc. 7370 (1965) from1-methoxy-pentafluorocyclobutyl-1-ene and elementary fluorine followedby hydrolysis of the methylcyclo-butyl ether) are reacted with K₂ CO₃analogously to Example 1b).

Yield: 41 g of Perfluorobutyl vinyl ether=74% of the theory

Bp_(ND) : 59°-60° C.

Amount of 1,1,1,2-tetrafluoroethyl-perfluorocyclobutyl ether <3%according to GC/MS.

EXAMPLE 3

Preparation of a Vinylidene fluoride/perfluoro-(cyclopentyl vinyl ether)copolymer

250 ml of deionised water were introduced into a 0.7 litre autoclave.3.9 g of sodium perfluoro-octanoate and 3.0 g of potassiumperoxydisulphate were then dissolved therein. The resulting solution wasadjusted to a pH of about 10 by means of sodium hydroxide. The sealedautoclave was then subjected three times to a nitrogen pressure of 10bar and subsequently released to normal pressure.

36 g of perfluoro-(cyclopentyl-vinyl ether) and 64 g of vinylidenefluoride were introduced into the autoclave and the reaction mixture washeated to 50° C. with stirring. After a reaction time of one hour atthis temperature, 50 ml of an aqueous solution containing 1 g ofascorbic acid, 5 mg of iron(II) sulphate and 1.5 g of sodium hydroxidebegan to be pumped in at a uniform rate over a period of 10 hours. Atthe end of this time, during which the reaction pressure fell from 42bar to 13 bar, the contents of the autoclave were cooled and unreactedgas mixture was ventilated off. The reaction mixture thus obtained waspoured into 300 ml of a 4% aqueous magnesium sulphate solution forcomplete coagulation. The product was washed with water and then dried.81 g of a white powder was obtained which was identified as a copolymerconsisting of units of vinylidene fluoride and perfluoro(cyclopentylvinyl ether). The copolymer is soluble in dimethyl formamide anddimethyl acetamide. The limiting viscosity is 0.2 dl/g (DMF, 25° C.).The molar ratio of vinylidene fluoride to perfluoro-(cyclopentyl-vinylether) in the copolymer was determined by ¹⁹ F nuclear resonancespectroscopy and found to be 91:9.

The following chemical shifts were found and evaluated in dimethylformamide-d₇ against trifluoroacetic acid as standard: ##STR3## a: -14;-16; -17; -46 ppm b: -42 ppm

c: -43 ppm

d: -52 ppm

e,e': -30; -35; -53; -59 ppm

f,f': -32; -38; -49; -55 ppm

COMPARISON EXAMPLE 3

Preparation of a vinylidene fluoride/perfluoro-(n-propylvinyl ether)copolymer

32 g of perfluoro-(n-propyl-vinyl ether) and 68 g of vinylidene fluoridewere copolymerised by a method analogous to that described in Example 3.72 g of a copolymer consisting of units of vinylidene fluoride andperfluoro-(n-propyl-vinyl ether) were isolated. The copolymer is solublein dimethyl formamide and dimethyl acetamide. The limiting viscosity is0.81 dl/g (DMF, 25° C.). The chemical composition was determined by ¹⁹F-nuclear resonance spectroscopy. The molar ratio of vinylidene fluorideto perfluoro-(n-propyl-vinyl ether) is 91:9.

EXAMPLE 4

110 ml of deionised water were introduced into a 0.3 litre autoclave.1.8 g of sodium perfluoro-octanoate were dissolved therein. Thissolution was adjusted to a pH of about 10 with sodium hydroxide. Thesealed autoclave was then three times and subsequently released tonormal pressure. 9 g of perfluoro-(cyclopentyl-vinyl ether) and 21 g ofvinylidene fluoride were introduced into the autoclave and the reactionmixture was heated to 80° C. with stirring. When this temperature hadbeen reached, 20 g of an aqueous solution containing 0.7 g of ammoniumperoxy disulphate were forced into the autoclave. After a total reactiontime of 7 hours during which the reaction pressure fell from 38 bar to15 bar, the autoclave contents were cooled and the unreacted gas mixturewas ventilated off. The emulsion thus obtained was poured into 130 ml ofa 4% aqueous magnesium sulphate solution for complete coagulation. Theproduct was washed with water and then dried. 19 g of a copolymer (whitepowder) consisting of units of vinylidene fluoride andperfluoro-(cyclopentyl-vinyl ether) were obtained. The copolymer issoluble in dimethyl formamide and dimethyl acetamide. The limitingviscosity is 0.2 dl/g (DMF, 25° C.). The chemical composition wasdetermined by ¹⁹ F-nuclear resonance spectroscopy. The molar ratio ofvinylidene fluoride to perfluoro-(cyclopentyl-vinyl ether) is 93:7.

COMPARISON EXAMPLE 4

9.5 g of perfluoro-(n-propyl-vinyl ether) and 20.5 g of vinylidenefluoride were copolymerised by a process analogous to that described inExample 4. 19 g of a copolymer consisting of units of vinylidenefluoride and perfluoro-(n-propyl-vinyl ether) were isolated. Thecopolymer is soluble in dimethyl formamide and dimethyl acetamide. Thelimiting viscosity is 0.7 dl/g (DMF, 25° C.). The chemical compositionwas determined by ¹⁹ F-nuclear resonance spectroscopy. The molar ratioof vinylidene fluoride to perfluoro-(n-propyl-vinyl ether) is 92:8.

DSC- and thermogravimetric (TGA) analyses were carried out on thecopolymers prepared in accordance with the above-mentioned Examples.

Measurements

TGA--Measuring apparatus TGS-2 (Perkin-Elmer); heating from roomtemperature to complete decomposition at the rate of 20K/min undernitrogen.

DSC--Measuring apparatus DSC-2 (Perkin-Elmer); heating twice from -50°C. to +200° C. at the rate of 20 K/min under helium (measured valuesobtained from the 2nd heating).

                  TABLE 1                                                         ______________________________________                                        Ex-   T.sub.g /°C.                                                                   T.sub.m /°C.                                                                   Temp./°C. for weight reduction by                ample (DSC)   (DSC)   1%   2%   3%   5%   10%  >90%                           ______________________________________                                        3     -8      162     280  354  373  389  406  800                            V-3   -28.5   155     240  250  267  345  381  580                            4     -14     150     142  203  228  271  338  900                            V-4   -33     148     111  164  213  261  402  672                            ______________________________________                                    

EXAMPLE 5

130 ml of deionised water were introduced into an 0.3-litre autoclave.0.5 g of lithium perfluorooctane sulphonate were dissolved therein. Thissolution was adjusted to a pH of about 10 with lithium hydroxide. Thesealed autoclave was then subjected three times to a nitrogen pressureof 10 bar and subsequently released to normal pressure. 9.3 g ofperfluoro-(cyclopentyl-vinyl ether) and 30 g of chlorotrifluoroethylenewere introduced into the autoclave and the reaction mixture was heatedto 90° C. with stirring. After this temperature had been reached, 20 gof an aqueous solution containing 0.8 g of potassium peroxydisulphatewere forced into the autoclave. After a total reaction time of 1.5 hoursduring which the reaction pressure fell from 19 bar to 15.5 bar, thecontents of the autoclave were cooled and the unreacted gas mixture wasventilated off. The reaction mixture thus obtained was poured into 130ml of a 4% aqueous magnesium sulphate solution for complete coagulation.The product was washed with water and then dried. 7.5 g of a copolymerconsisting of units of chlorotrifluoroethylene andperfluoro-(cyclopentyl-vinyl ether) were obtained. The copolymer is notsoluble in dimethyl formamide or dimethyl acetamide.

The following copolymer composition was determined by analyses of thechlorine and fluorine contents:

Chlorotrifluoroethylene/perfluoro-(cyclopentyl-vinyl ether)=95/5 (molarratio).

EXAMPLE 6

8.9 g of perfluoro-(cyclopentyl-vinyl ether) and 60 g ofchlorotrifluoroethylene were copolymerised for 2 hours at 70° C. by aprocess analogous to that described in Example 5. 26 g of a white powderwere isolated. This was identified as a copolymer consisting of units ofchlorotrifluoroethylene and perfluoro-(cyclopentyl-vinyl ether).

The following copolymer composition was determined with the aid ofanalyses of the chlorine and fluorine content:Chlorotrifluoroethylene/perfluoro-(cyclopentyl-vinyl ether)=98/2 (molarratio).

COMPARISON EXAMPLE TO EXAMPLE 6

15 g of the perfluoro-(cyclopentyl-vinyl ether), prepared according toComparison Example 1b, which could only be obtained in 55% purity bythis variation of the process, and 60 g of chlorotrifluoroethylene werecopolymerised for 6 hours at 70° C. by a process analogous to thatdescribed in Example 6. In addition to 0.2 g of a beige colouredcoagulate, 11 g of an ochre coloured powder were isolated from theemulsion which contained large quantities of unidentified impurities inaddition to the units of chlorotrifluoroethylene andperfluoro-(cyclopentyl-vinyl ether) bound in the copolymer.

    ______________________________________                                                Example 6  Comparison Example 6                                       ______________________________________                                        Yield     43% in 2 h   22% in 6 h                                             Appearance                                                                              white powder ochre coloured powder +                                                       beige coloured                                                                coagulate                                              ______________________________________                                    

We claim:
 1. Thermoplastic fluorocopolymers obtained by thecopolymerisation ofa) from 99.5 to 50 mol-% of at least one ethylenehaving 1 to 4 fluorine atoms and b) from 0.5 to 50 mol-% of at least oneperfluoro-(cycloalkyl-vinyl ether) corresponding to Formula I ##STR4##wherein n=0, 1 or 2 andm=3, 4 or 5 and c) from 0 to 40 mol-% of at leastone other comonomer selected from ethylene, propylene and straight chainor branched C₃ - to C₈ -alkenes containing at least one fluorine atom.2. A process for the production of thermoplastic fluorocopolymersaccording to claim 1, characterised in that theperfluoro-cycloalkylvinyl ether of formula I is prepared from thecorresponding 2-cycloalkoxy-propane carboxylic acid fluoridescorresponding to the following formula II ##STR5## by slowly heatingthese to a temperature of from 110°-140° C. in a suitable solventcontaining a salt-forming agent in the presence of a catalytic quantityof N,N-dimethylformamide, withdrawing a distillate at a temperaturestarting from 100°-120 ° C. corresponding to a condensation temperatureof from 60° to 90° C. and redistilling the distillate at normalpressure.